Conventionally, the axle driving system of integral hydraulic transmission has been well known as disclosed in, for example, the U.S. Pat. Nos. 4,914,907, 4,932,209 and 4,891,943. In the above-mentioned prior arts, an axle casing for supporting axles is made dividable into an upper axle casing and a lower axle casing from the position of the fixedly junction surface including a lengthwise axis of the axle, and an L-like-shaped center section is fixed in the axle casing, and the upper surface of the horizontal portion of L-like-shaped center section is served as the pump mounting surface and the outside surface of the vertical portion perpendicular to the horizontal portion of the same is served as the motor mounting surface. However, the pump mounting surface is positioned to coincide with the plane of the fixed Junction surface of the axle casing, whereby a pump shaft is positioned high and a belt transmitting mechanism for inputting an engine power is positioned high, thereby making it difficult to lower a platform of the vehicle.
At the center section are provided a closed circuit for circulating operating oil between a hydraulic pump and a hydraulic motor and charge ports for supplying to the closed circuit the operating oil filtered to be stored at the bottom of axle casing. Since such center section is contained in the axle casing, for the purpose of filling the operating oil into the closed circuit to put the hydraulic transmission in the state of being operable during the assembly, there is no way other than waiting that the hydraulic pump is driven to naturally take the operating oil into the charge port, which takes much time and is ineffective.